A number of markets are introducing MSS (Mobile Soft Switch) with IP (Internet Protocol) transport while keeping a large part of their TDM (Time Division Multiplex) network untouched for some years to come. As a result, there is a co-existence of IP and TDM transport for Mobile Circuit Core. When an IP network is congested or not available, operators require that, if possible, an alternative TDM route is used instead of simply rejecting the call.
It is possible to handle calls via an alternative TDM network, if a BICC (Bearer Independent Call Control) route is congested, as one can re-select an ISUP (ISDN User Plane) route with TDM transport as an alternative. When a BICC route is up-and-running and the problem is related to the User Plane, the call is rejected by M-MGW (Mobile Media Gateway) e.g. using MBAC (Measurement Based Admission Control) or similar mechanism depending upon the conditions of the User Plane. The term MGW (Media Gateway) is used throughout this text as a non-limiting example for a node which handles the user plane of a call and comprises resources for the user plane handling. If the call is handled by single server, e.g. a Mobile Services Switching Center (MSC) it is still possible for the server to re-route the call to an alternative route. For the case of calls via two or more servers, which is more common, it is too late in the call set up to re-route the call and the call will fail.
A solution is required from markets to improve the situation from several customers.
Existing solution are not good enough for real market issues since:                When Transport Network is congested, BICC routes are usually still up and running, i.e. a solution is preferable if it does not only consider a scenario in which signaling transport is also congested and hence a BICC route is not available.        Typically, long distance calls are controlled by two servers, i.e. a solution is preferable if it does not only consider a scenario in which the call is controlled by a single MSC-S (Mobile Services Switching Center Server).        
The above is an example of a situation in which a communication system has routing capabilities over two different types of networks.